Gradient Cross-linking of Poly (dimethyl) siloxane and Associated Droplet Wetting

Efficient and facile fabrication of a substrate with elasticity gradient has remained an area of contemporary importance due to its potential in a plethora of biological applications such as adhesion and proliferation of cancerous cells etc. However, the existing methods suffer from the use of sophisticated setup and multiple steps involving costly equipment. In this work, a novel straightforward method has been used to impart a temperature gradient (~ 77°C to 144°C) during cross-linking, resulting in a gradient of Young's modulus (~1.21 MPa to 2.38 MPa). The phenomenon is attributed to the fact that higher temperature stimulates the hydrosilylation reaction at the hotter side leading to extensive cross-linking. However, the colder side, due to slower hydrosilylation, allows a relative slip among the polymer chains and helps to dissipate the stress resulting in the lower value of Young’s modulus. The wetting characterization shows the increasing hydrophobic behavior (103° to 117°) towards the softer side. This is attributed to the increasing deformation of the substrate at the three-phase contact line due to elastocapillary interaction.